The chemical structures and biological activities of indole diterpenoids

Indole diterpenoids (IDTs) are an essential class of structurally diverse fungal secondary metabolites, that generally appear to be restricted to a limited number of fungi, such as Penicillium, Aspergillus, Claviceps, and Epichloe species, etc. These compounds share a typical core structure consisting of a cyclic diterpene skeleton of geranylgeranyl diphosphate (GGPP) and an indole ring moiety derived from indole-3-glycerol phosphate (IGP). 3-geranylgeranylindole (3-GGI) is the common precursor of all IDTs. On this basis, it is modified by cyclization, oxidation, and prenylation to generate a large class of compounds with complex structures. These compounds exhibit antibacterial, anti-insect, and ion channel inhibitory activities. We summarized 204 compounds of IDTs discovered from various fungi over the past 50 years, these compounds were reclassified, and their biological activities were summarized. This review will help to understand the structural diversity of IDTs and provide help for their physiological activities. Graphical Abstract


Introduction
IDTs are a structurally diverse class of fungal secondary metabolites, all sharing a common core structure consisting of indole, and a diterpene carbon backbone derived from four mevalonate-derived isoprene units [1]. The molecular complexity of these compounds is achieved by adding more isoprene units to the core structure and by various modifications such as oxidation, cyclization, and halogenation. IDTs are ubiquitous in the natural environment, and moldy food produced by Penicillium sp. is a common source of these compounds. For example, the fungus P. tularense found in tomatoes has metabolites such as janthitrems, paspalinine, paxilline, etc. [2]. P. crustosum, which produces the shivering mycotoxin penitrems, is a common foodborne fungus that can cause the spoilage of many foods [3].
Most of the IDTs are potent tremor mammalian mycotoxins [4], that also exhibit excellent biological activities, including cytotoxic, antibacterial, antiviral, and protein tyrosine phosphatase inhibitory activities [5]. To date, some IDT compounds have been used for drug discovery. For example, as BK channel blockers, IDTs have been shown to reduce intraocular pressure and have been used to treat glaucoma [6]. The H1N1 virus is an invasive strain of influenza virus that can cause death in humans, and many IDTs have also shown significant activity against the H1N1 virus, especially emndole SB [7]. In agricultural settings, there has been a general trend toward using tremor-free IDTs as pesticides, such as 20,25-dihydroxyaflavinine [8].
According to previous reports, 3-GGI is a common precursor compound of all IDTs. The origin of the indole ring in its structure was clarified in 1983, when Jesus et al. studied the biosynthesis of penitrem A, the results of isotope labeling experiments showed that the indole part was derived from the IGP precursor of tryptophan [9]. In 2013, Tagami et al. analyzed a pentenyltransfer PaxC in paxilline biosynthetic gene cluster, and proved that indole ring derived from IGP through in vitro enzyme activity experiment [10]. Next, on this basis, IDT can be further divided into two types, namely the paspaline type with a large proportion and a small part of the non-paspaline type. There are some reviews related to IDTs have been presented. For example, the synthesis and activity of paspaline-type compounds [11,12], the structural diversity and biological activity [5], biosynthesis of IDTs are described [13][14][15]. However, considering that these reviews do not classify these two types of compounds uniformly, and exclude the cover of the latest IDT compounds in recent years. So here, we renamed the IDT skeleton rings A, B, C, D, E, and F, and 77 non-paspaline skeleton and 127 paspaline skeleton IDTs were uniformly reclassified and summarized according to their structures and oxidative modifications. This review will contribute to the scientific community's comprehensive and compact understanding of the complex and diverse IDTs.

Emindole SB series
The emindole SB series is different from the Nodulisporic acid series in that there is no caproic acid on the E ring ( Fig. 2 and Table 2). In 1966, the compound emindole SB (32) was isolated from Claviceps paspali, which was cytotoxic to cancer cell lines, and also showed antibacterial activity against Staphylococcus aureus ATCC 6538 and Bacillus subtilis ATCC 6633 [23][24][25]. In 2010, asporyzin C (33) was isolated from Aspergillus oryzae, and the antibacterial activity against E. coli as well as the antifungal activity against plant pathogens Colletotrichum lagenarium and Fusarium oxysporium were assayed. 33 exhibited intense activity against E. coli with an inhibitory diameter of 8.3 mm [25]. In 2020, the natural product penerpene J (34) was found in the fungus Penicillium sp. KFD28. This compound has inhibitory activity against both PTP1B (protein tyrosine phosphatase 1B) and TCPTP (protein tyrosine phosphatase), with IC 50 values of 9.5 μM and 14.7 μM, respectively [26]

Emindole DA series
The common feature of this series of compounds is that they contain a 6/6-membered ring linked to a methylene group at the 3-position of the indole ring ( Fig. 3 and Table 3). In 1988, the X-ray molecular structures of emindole DA (38) and DB (39) from Emericella desertorum were reported, both of which are tremor toxic to mammals [28,29]. In 1989, nominine (40) was isolated as the leading organic soluble component of the sclerotium of the fungus Aspergillus nomius NRRL 13,137, which showed potent activity against the widespread crop pest Heliothis zea. When added to the standard test diet at 100 ppm dry weight, it resulted in 40% mortality and 97% weight loss relative to controls [30]. In 1992, compounds  [34]. In 2021, the compound penerpenes M (51) was discovered from the fungus Penicillium sp. KFD28. However, no antibacterial activity was found [35].

Aflavinine series
This series difference from the emindole DA series is that the 3-position of the indole ring is directly connected with the 6/6-membered ring ( Fig. 4 and Table 4).

Tubingensin A series
The structure of this series is characterized by the presence of a benzene ring attached to the indole ring B ( Fig. 5 and Table 5). In 1989, tubingensin A (58) and its structural isomer tubingensin B (59) were isolated from the fungus Aspergillus tubingensis by Gloer JB and colleagues, and 58 was found to be resistant to the general crop pest Heliothis zea, and exhibit showed in vitro antiviral activity against herpesvirus type I [38], while 59 showed mild activity against the crop pest H. zea, resulting in a 10% mortality rate when added to a standard diet at 125 ppm. The compound also showed almost identical activity to 58 in assays against herpes simplex virus type I with an IC 50 of 9 μg/mL, but was more cytotoxic to HeLa cells (IC 50 4 μg/mL) [39]. In 1990, the compound aflavazole (60) was isolated from Aspergillus flavus. When added at 100 ppm to the standard test diet, 60 showed significant feeding-rejecting activity against the fungus-eating beetle Carpophilus hemipterus and was second only to dihydroxyaflavinine in activity against C.
hemipterus among the IDT mycorrhizal metabolites of A. flavus [40]. When added to diets at concentrations found in A. flavus sclerotia (200-600 ppm), almost complete feeding deterrence was observed [40,41]. In 2019, Miles CO and his colleagues isolated the compound shearilicine (61) from the strain Penicillium sp. ZO-R1-1, which had an IC 50 value of less than 10 μM against L5178Y or A2780 cells, was tested against the human embryonic kidney cell line HEK-293. The results showed the highest selectivity in tests with SI (selectivity index) values in the range 3.3-8.1 and were also the most active metabolite against L5178Y cells with an IC 50 value of 3.6 μM and A2780 cells with an IC 50 value of 8.7 μM [42].

Other non-paspaline skeleton type compounds
This series contain irregular non-paspaline type compounds ( Fig. 6 and Table 6). In 1992, the compound paxinorol (62), isolated from the fungus Penicillium paxilli, which was found to be toxic to mammals, and it reduced the activity behavior of mice, but returned

A-ring prenylation
Based on the paspaline skeleton, the modification of isopentenyl was added to the 20, 21 or (and) 22 positions of the A ring of its indole ring ( Fig. 9 and Table 9). In 1964, Wilson BJ isolated the compounds ɑ-aflatrem (126) and β-aflatrem (127) from Aspergillus flavus, which are fibrillating mycotoxins with acute neurotoxic effects [74,75]. In 1977, Cole RJ et al. discovered the compound paspalitrem A (128) from Claviceps paspali, a toxin that can vibrate muscles [76,77]. In 1992, compounds sulpinine A (129) and B (130) were isolated from Aspergillus sulphureus, both of which were active against H. zea but not against C. hemipterus [44]. Among them, 129 have the most potent activity. When this compound was added to the standard test diet at 100 ppm, a 96.0% reduction in body weight gain compared to the control was noted after one week, and a 10% mortality rate was also observed in this assay. 130 brought a similar weight gain reduction of 87.2%. Moreover, 129 was also cytotoxic to human lung cancer A549, breast cancer MCF7, 102 Epipaxilline Anti-cancer Penicillium sp. [26] and colon adenocarcinoma HT-29 cells with ED 50 values of 25.7, 58.1, and 3.7 µg/ mL [44,78]. In 1995, Tomoda H et al. isolated and characterized terpendole L (131) from the culture broth of Albophoma yamanashiensis by using different production media [62]. This compound has a moderate inhibitory effect on ACAT activity with an IC 50 value of 32.4 μM in rat liver microsomes [62][63][64].

Ascandinine B
A. candidus [53] 125 Ascandinine D Cytotoxicity A. candidus [53] isolated new compounds, termed voluhemins A (154) and B (155), from the culture broth of the fungal strain Volutella citrinella BF 0440. They have a common IDT core and two additional isoprenyl moieties, and 155 are O-methylated 154. 154 can inhibit the activities of SOAT1 and SOAT2 with a SI value of + 0.45, and 155 can selectively inhibit the SOAT2 isoenzyme. However, none of which is cytotoxic [83].
When the mouse macrophage cell line RAW264.7 was pretreated with this compound for 2 h before LPS stimulation, it inhibited NO production by 40% at 10-30 μg/ ml with no toxicity [93]. In 2018, new compounds 11,12-epoxyjanthitrem B and 11,12-epoxyjanthitrem C were isolated from the fungus Penicillium janthinellum,

A-ring with 5/6 member ring
The difference from the previous type is that this type is further oxidatively cyclized into a 5/6-membered ring based on the diprenyl modification at the 20 and 21 positions of the A ring of the indole ring ( Fig. 12 and Table 12). In 1981, two strong neurotoxins, lolitrems A (182) and B (183), were isolated from herbs that developed a livestock disease known as "ryegrass staggered disease." They can poison livestock with tremors that do not directly impair spatial learning and memory, but reduce voluntary movements in poisoned animals; later, perennial ryegrass toxicosis (PRGT) was prevented by limiting the concentration of 183 [95]. In 1992, lolitriol (184) was found in extracts of endophyte-infected ryegrass leaves and cultures of A. lolii [43]. Moreover, 183 is quickly degraded to compound 184, which does not cause tremors even at 20 mg/kg, so its activity is at least 20-fold lower than 183 [96]. In 1994, Christopher et al. obtained the abundant secondary compound lolitrem E (185) when 183 was purified from ryegrass staggers (RGS), which has intense BK channel activity but no tremor effect in animals [97,98]. In 1996, Sarah et al. isolated lolitrem F (186), a stereoisomer of the vibratory mycotoxin 183, from ryegrass infected with Acremonium Lolii. The compound 186 was found to have similar potency and duration of action as 183 in standard mouse bioassays, but was slightly less active than 183 [99]. In 1997, the compound lolitrem H (187) was discovered [71]. In 1997, Sarah et al. isolated lolilline (188) from an extract of ryegrass seeds infected with the endophytic fungus Acremonium lolii, which does not have tremor effects [100]. In 1998, lolitrem N (189), lolicine A (190), and B (191) were identified in an extract of perennial ryegrass (Lolium perenne) seeds infected with the endophytic fungus Neotyphodium lolii, and they are lolitremlike compounds [101].